1. Field of the Invention
The present invention relates to a technology for linearizing the input/output characteristic of a power amplifier.
2. Description of the Related Art
In mobile radio communications, since a signal in an unused band interferes with an adjacent channel, it is necessary to secure the linearity of a power amplifier in order to suppress the signal power in an unused band to a lower level. Although conventionally an output back-off method for suppressing an output power in such a way that out-of-band power becomes sufficiently low is used for this purpose, it is difficult to improve its linearity while securing the efficiency since this method has a trade-off between the efficiency and the linearity.
FIG. 1 shows the input/output characteristic of a typical power amplifier.
The output back-off method is described with reference to FIG. 1.
As the output of a power amplifier becomes high, as shown in (1) of FIG. 1, the relationship between the input and output of the power amplifier deviates from a linearity relationship, as shown in (2) of FIG. 1. Therefore, in the output back-off method, only the low input part of the characteristic curve (1) of an actual power amplifier in which the linearity is secured is used. Since as shown in FIG. 1, the low input part of the characteristic curve of a power amplifier yields an almost ideal characteristic curve (2), sufficient linearity can be obtained from the operation of a power amplifier, and the occurrence of an RF element due to the degradation of the waveform of a signal to be amplified, specifically the occurrence of noise in an adjacent channel can be suppressed to a low level.
However, since if the input of a power amplifier is low, the amplification efficiency of the power amplifier becomes low and the power amplifier cannot be used efficiently enough, which is a problem. In order to improve the amplification efficiency, the input must be increased, which leads to non-linearity in the power amplifier. Thus, since the output back-off method has a trade-off between the efficiency and the linearity, it is difficult for the high efficiency and linearity to exist together.
As a method for improving the linearity while securing the amplification efficiency of a power amplifier, a linearization by a pre-distortion method is attempted. For detailed information about this method, please see the specification of Japanese Patent Application 9-297297 previously applied for by this applicant.
FIGS. 2A and 2B are drawings showing a pre-distortion method.
As shown in FIG. 2A, if a pre-distortion is performed for the input/output characteristic of a power amplifier (PA), a baseband I signal and a baseband Q signal are inputted to a baseband variable attenuator 180, the power of baseband signals are adjusted, and the baseband signals are inputted to a pre-distortion circuit 181. In the pre-distortion circuit 181, the low input part of an inputted signal in which the linearity shown in the curve (1) of FIG. 1 is pre-processed using a function adjusted in such a way that the inclination of the characteristic curve can be maintained, based on the inverse function of the input/output characteristic of a power amplifier 183 as shown in the curve of FIG. 1. Specifically, no special treatment is made for the inputted baseband signals while the input value of the input baseband signals are within the range where the input/output characteristic of the power amplifier 183 shows linearity. However, if the input/output characteristic is out of the range where the input/output characteristic of a power amplifier 183 shows non-linearity, the power value of the inputted baseband signals are increased, and the degradation of the signal waveform due to the increase of the non-linearity of the power amplifier 183 is cancelled. The baseband signals pre-distorted by the pre-distortion circuit 181 are inputted to a modulator (in FIG. 1, a WPSK modulator 182) and QPSK-modulated. By QPSK modulation, the pre-distorted signals are converted to RF signals, are amplified by the power amplifier 183, and are transmitted from an antenna 184. It is because a pre-distortion is performed by a digital signal process that the pre-distortion circuit 181 is installed immediately before the QPSK modulator. Specifically, if the pre-distortion circuit 181 is installed immediately after the QPSK modulator, signals are converted to RF signals and it becomes difficult to pre-distort the signals by a digital process.
If the pre-distortion method is realized using a digital circuit and if a wide power range is covered only by a digital signal process with a configuration as shown in FIG. 2A, the quantization error increases at a low power, and an adjacent channel leak power (ACP) characteristic significantly degrades. Specifically, the values of a function for applying a pre-distortion function, such as Fpd (x) shown in FIG. 4 are discretized, and if those values are stored in a table and a value corresponding to an inputted signal value is read from the table, the discretization against the inclination of the function of a signal corresponding to the low signal input part of the function with a gentle inclination becomes too steep, and a pre-distortion with a large error is applied against an input signal value changing like as analog signal.
FIG. 2B shows an adjacent channel leak power (ACP) against the output of the power amplifier 183. In FIG. 2B, the vertical axis and horizontal axis are ACP (unit: dB) and the output of the power amplifier 183 (unit: dBm), respectively.
If a pre-distortion is not performed, as indicated by 0 and + in FIG. 2B, it has been found that as the output power Pout of the power amplifier increases, the adjacent channel leak power (ACP) increases. However, ACPs in the case where a pre-distortion is performed are indicated using a variety of lines (a dotted line, a one-point chained line, etc.,). Each line shown in FIG. 2B indicates with how many bits a pre-distortion function is digitized (discretized), specifically, cases of 7 bits, 8 bits, 9 bits, 10 bits, 11 bits and 12 bits are indicated from the top. As shown in FIG. 2B, as the number of bits to be used when a pre-distortion function is digitized increases, the ACP is further improved.
Thus, although increasing the number of input/output bits to be used when a pre-distortion function is discretized, is considered in order to improve an ACP, in that case, the circuit scale becomes large.
Although in the configuration shown in FIG. 2B, the attenuation characteristic of a variable attenuator (VATT) 180 is not taken into consideration, the attenuation of the variable attenuator further degrades the ACP, which is another problem.
FIGS. 3A and 3B show the configuration and characteristic in the case where compensation is made by pre-distortion and where the attenuation characteristic of a variable attenuator is also taken into consideration.
According to the configuration shown in FIG. 3A, a pre-distortion is performed for baseband I and baseband Q signals in a pre-distortion circuit 190, and the signals are modulated in a QPSK modulator 191. Then, the signal power of the signals is attenuated in a variable attenuator 192, is amplified in a power amplifier PA 193 and the RF signals are transmitted from an antenna 184. At this time, an attenuation-degree control signal, VATT control provided to the variable attenuation 180 is inputted to the pre-distortion circuit 190, and a pre-distortion is applied taking into consideration the attenuation characteristic of the variable attenuator 192.
FIG. 3B shows the relationship between an output value to a PA 193 and an ACP in the configuration shown in FIG. 3A.
The meaning of each curve shown in FIG. 3B is the same as that shown in FIG. 2B. As is clearly seen from FIG. 3B, if a pre-distortion is applied, an ACP characteristic is further improved than when the pre-distortion not applied. The greater the number of bits to be used when a pre-distortion function is digitized, the more the ACP characteristic is improved. If an ACP characteristic in the case where a pre-distortion is applied, as shown in FIG. 2B, is compared with an ACP characteristic in the case where a pre-distortion is applied, as shown in FIG. 3B, it is found that the ACP characteristic in a configuration where the attenuation characteristic of the variable attenuator is also taken into consideration is more improved, as shown in FIG. 3A.
Therefore, if an output is adjusted by the variable attenuator 192 in a configuration as shown in FIG. 3A, the performance can be secured with fewer input/output bits, as shown in FIG. 3B. However, in such a configuration, since it becomes necessary to switch over pre-distortion information according to the attenuation level of the variable attenuator 192, as a result, the circuit scale cannot be reduced dramatically.
In particular, since in a case where this method is applied to a mobile radio terminal, both a small size and light weight and a low power consumption becomes worthwhile targets, the circuit scale must be reduced by contracting pre-distortion information by a simple method for preventing the circuit scale from increasing.
The objective of the present invention is to provide an apparatus and method for making a high-accuracy pre-distortion without the increase of the circuit scale.
The pre-distortion apparatus of the present invention for linearizing the input/output characteristic of a power amplifier comprises a differential pre-distortion unit using a differential between a pre-distortion function to linearize the input/output characteristic of the power amplifier and a function expressed by a prescribed equation as a differential pre-distortion function in the pre-distortion apparatus and generating a differential pre-distortion signal for an inputted signal using the differential pre-distortion function, and a combination unit combining the differential pre-distortion signal with the inputted signal and outputting the combined signal as a signal after pre-distortion.
The pre-distortion method of the present invention for linearizing the input/output characteristic of a power amplifier comprises the steps of (a) using the differential between a pre-distortion function to linearize the input/output characteristic of the power amplifier and a function expressed by a prescribed equation as a differential pre-distortion function in the pre-distortion apparatus and generating a differential pre-distortion signal for an inputted signal using the differential pre-distortion function, and (b) combining the differential pre-distortion signal with the inputted signal and outputting the combined signal as a signal after pre-distortion.
According to the present invention, since a pre-distortion function with a steep inclination is converted to a differential pre-distortion function which is obtained using the differential between the function and another appropriate function, and the converted differential pre-distortion function is digitized and stored, information about the differential pre-distortion function can be accurately digitized using a small number of bits when the function table of the differential pre-distortion function is generated.
Therefore, when a pre-distortion is performed using this function table, the quantization error can be reduced and simultaneously the memory capacity of a memory device for storing the function table can be reduced since the number of bits required to store the information can be reduced. Thus, the circuit scale of a pre-distortion circuit can be reduced. Accordingly, a high-accuracy pre-distortion apparatus can be realized with a small-scale configuration.